1. Field of the Invention
The present invention relates to a method for discriminating an anomaly in gas composition of a laser gas excited by discharge in a discharge tube, and to a discharge excitation type gas laser oscillator using the same.
2. Description of the Related Art
Generally, an example of a conventional high-frequency discharge excitation type gas laser oscillator, in which laser light is generated by discharge excitation of a laser gas in a discharge tube, has been known from the disclosure of Japanese Patent Publication No. H07-221378 A1. A gas laser oscillator as disclosed in Japanese Patent Publication No. H07-221378 A1 has a laser head for generating laser light, a high-frequency power supply unit for supplying electric power to a discharge tube of the laser head, and a controller unit for controlling the high-frequency power supply unit. The gas laser oscillator has the capability of discriminating an anomaly of a discharge load in the laser head. Thus, at the start or during the operation of the high-frequency power supply unit, output data such as current, voltage, and frequency of the high-frequency power supply unit are compared with respective normal values, and based on the result of the comparison, it is discriminated whether or not the discharge in the discharge tube has been started. The discharge load is diagnosed as abnormal, for example, when the discharge has not been started. However, in such a gas laser oscillator, if an anomaly is discriminated during the operation of the high-frequency power supply unit, when there is actually an anomaly in the discharge load, excessive voltage may be applied to the discharge load or the output current may become excessively large, so that the discharge tube or the high-frequency power supply unit may be destroyed. In addition, depending on the conditions for discriminating an anomaly in the discharge load, for example, when the pressure of the laser gas is high, the discharge tube or the high-frequency power supply unit may be destroyed even at the start of the high-frequency power supply unit.
As another example of prior art, Japanese Patent Publication No. S62-60837 B1 discloses a laser apparatus, in which deterioration of laser gas is discriminated by calculating the oscillation efficiency that is calculated based on input current and output current of a laser. In this example of prior art, when the gas pressure is high at the time of discriminating the deterioration of the laser gas, the voltage applied to the discharge load or the output current may become excessively large in the same manner as described above, and may destroy the discharge tube or the high-frequency power supply unit.
As still another example of prior art, a method is disclosed in Japanese Patent Publication H05-291650 A1 (Patent No. 2612659), in which, apart from a laser container for generating laser light, a separate container is provided for detecting gas deterioration so that gas deterioration can be detected by measuring discharge voltage and discharge current therein. In this case, a separate container for detection and a separate electrode need to be provided for detecting gas deterioration. A method is also disclosed in Japanese Patent Publication No. 2000-286494 A1, in which the progressive state of deterioration of a mixed gas in a chamber is discriminated based on a spontaneous emission light detected at the time of discharge. Similarly, Japanese Patent Publication No. H02-278887 A1 discloses a method in which gas deterioration is detected by measuring the intensity of light in a discharge tube by using a photo-sensor and an optical filter for transmitting a light having a peculiar wavelength. Also, Japanese Patent Publication No. 2001-44534 A1 discloses a method and an apparatus for measuring the DC-breakdown voltage of a gas mixture in a laser gas discharge chamber by means of a breakdown voltage detecting unit and adjusting the composition of a discharge gas mixture by using the information on the DC-breakdown voltage. Further, a method is also disclosed in Japanese Patent Publication No. 2001-242077 A1, in which a Fourier Transform Infrared Spectrometer or the like is used to detect and discriminate an infrared absorption spectrum of infrared ray generated in a discharge.
A method is also disclosed in Japanese Patent Publication No. 2002-208746 A1, in which, in order to know the state of a laser gas, a high voltage and temperature are measured, and using a discrimination sequence based on multiple discrimination criteria, the composition of a laser gas in a chamber is maintained while measuring the rate of change of the discharge voltage and the gas temperature before and after gas injection. A system is disclosed in Japanese Patent Publication No. 2000-151002 A1, in which, in order to discriminate the state of gas deterioration, current data is compared with a plurality of master data which has been obtained in a discharge of a gas mixture of an optimum composition, and discrimination is made based on the comparison. In this example of prior art, a large number of data need to be measured and compared, and complicated operational processing and sequences, and a large number of data processings are required.
In the case where detorioration of a laser gas needs to be discriminated under high gas pressure, the voltage applied to the discharge load and the output current tend to become large, and therefore, the discharge tube and the high-frequency power supply unit may be destroyed. There is also another problem that a special measuring unit and apparatus may be required for analyzing the composition of the laser gas.